Add swing function

[Stazed]

See seq42

[jean-emmanuel]

I took a slightly different approach than seq42's in seq192, thought you might be interested : https://github.com/jean-emmanuel/seq192/commit/7703b0aee15f95a22a6f20cb4ca928795a172cf0

[Stazed]

Thanks, I will take a look at it.

[jean-emmanuel]

I changed the maths to make it more consistent and cpu-friendly, should work fine as of v1.5.0.